if the else short form

James Harris james.harris.1 at googlemail.com
Mon Oct 4 12:15:57 CEST 2010


On 29 Sep, 18:20, Seebs <usenet-nos... at seebs.net> wrote:
> On 2010-09-29, Tracubik <affdfsdfds... at b.com> wrote:
>
> > Hi all,
> > I'm studying PyGTK tutorial and i've found this strange form:
>
> > button = gtk.Button(("False,", "True,")[fill==True])
>
> > the label of button is True if fill==True, is False otherwise.
>
> > i have googled for this form but i haven't found nothing, so can any of
> > you pass me any reference/link to this particular if/then/else form?
>
> Oh, what a nasty idiom.
>
> Here's the gimmick.
>         ("False,", "True,")
> is a tuple.  That means you can index it.  For instance:
>         ("False,", "True,")[0]
> is the string "False,".
>
> So, what is the numeric value of "fill == True"?  Apparently, at least
> at the time this was written, it was 0 if fill was not equal to True,
> and 1 if fill was equal to True.
>
> Let me say, though, that I'm a C programmer, so I'm coming from a language
> where the result of 0-or-1 for test operators is guaranteed, and I still
> wouldn't use this in live code.  It's insufficiently legible.

I'm surprised you don't like this construct. I hadn't seen it until I
read the OP's question just now. However, it's meaning was immediately
apparent.

I should say where I'm coming from. Contrast the following C and
Python:

  text = fill == true ? "True," : "False,";   (C)
  text = ("False,", "True,")[fill == true]    (Python)

I never liked C's ?: construct partly because it doesn't scale well.
To accept more than two options it requires the programmer to build a
small hierarchy which can be awful to read and may be best expressed
as a separate function. I'd rather have a language change a predicate
to a small integer and use that to index a set of results - and this
is exactly what the OP's tutorial does.

I'm not saying you are wrong, merely expressing a different view and
explaining why.

As another hypothetical example where sgn() returns -1, 0 or +1

  position = ("less", "equal", "greater")[sgn(a - b) + 1]

Though where the list gets much longer it would be good to be able to
label the cases for legibility.

cc. comp.lang.misc

James



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